Performance of a linear interference canceller for a ds/cdma synchronous system based on the ekf delay estimator

PERFORMANCE OF A LINEAR INTERFERENCE CANCELLER FOR A DS/ CDMA SYNCHRONOUS SYSTEM BASED ON THE EKF DELAY ESTIMATOR Pila Diaz, Ram6n Agus i Dep . o Signal Theo y and Communica ions inke si a Poli kcnica de Ca alunya C/ So Eulilia d’hzizu s/n Mhdul D4 08034 Ba celona, Spain Tel. 34-3-4017201, Fax 34-3-4017200 e-mail : [email p o ec ed] Abs ac : Since he con en ional de ec o o en ails o p oduce eliable decisions o a CDMA channel, se e al new mul iuse de ec o s ha e been p oposed la ely. In his pape , we p opose bo h a simple linea scheme o in e e ence cancella ion, which exhibi s good pe o mance, and a synch onism scheme based on he Ex ended Kalman Fil e (EKF) o achie e synch oniza ion among he di e en use s a he base s a ion, o a di ec - sequence code-di ision mul iple-access (DSICDMA) synch onous sys em. The in luence o he synch onism scheme on he pe o mance o he in e e ence cancelle is analyzed unde Rayleigh mul ipa h ading. I. INTRODUCTION In mobile communica ion sys ems o nex gene a ion, he inc ease in capaci y will be one o he mos impo an equi emen s o be ul illed. In o de o imp o e he capaci y in a DS/CDMA cellula sys em, which is basically limi ed by in e e ence, known in o ma ion on in e e ing signals can be employed o elimina e hem a he base s a ion. The op imum demodula o o signals ha sha e he same channel in ime and equency, desc ibed in [l], is based on a mul iuse join de ec ion scheme. This op imum ecei e is, howe e , oo complex o be implemen ed in a eal mobile communica ion sys em, since i s g ows exponen ially in complexi y as he numbe o simul aneous use s o be de ec ed inc eases. 0-7803-2742-XI95 $4.00 0 1995 IEEE An al e na i e scheme based on in e e ence cancell a ion echniques is p esen ed he e o a synch onous DS/CDMA cellula sys em, which employs Gold codes as sp eading sequences. The use o quasi-o hogonal sequences as he Gold codes in a synch onous DS/CDMA sys em imp o es he pe o mance ob ained wi h an asynch onous access as a esul o he good beha iou o hese sequences in e ms o pe iodic c oss-co ela ion. Ne e heless, in a mul ipa h en i onmen , he quasi-o hogonali y be ween use s in he same cell is des oyed by he mul iple p opaga ion pa hs coming a he ecei e . In o de o keep on his quasi- o hogonali y, i is necessa y o eso o a cancella ion scheme ha elimina es he in e e ing p opaga ion pa hs, as shown in Fig. 1. In his case, he complexi y o he ecei e only g ows linea ly as he numbe o simul aneous use s wi hin a gi en cell inc eases. Despi e i s simplici y, i is shown in [2] ia some compu e simula ion ha his scheme imp o es signi ican ly he beha iou o he con en ional In e e ence cancelle l a a b Fig. 1 A DS/CDMA ecei e wi h an in e ie ence cancella ion scheme 68 ecei e in e ms o capaci y. To suppo his synch onous access among he di e en mobiles a he base s a ion, a simple p ocedu e based on a delay es ima ion scheme o low complexi y is illus a ed in Fig. 2. The p ocedu e we p opose consis s o es ima ing he channel delay o e e y mobile a he base s a ion, and acco ding o he ime e e ence e ained a he base, send he app op ia e command o each mobile in o de o make hem adjus hei ansmission ime o his e e ence. I his adjus men is done in s eps o A seconds, le us no e ha he synch onism command o be sen o e e y use would be made up o only one bi . The synch onism scheme his p ocedu e is based on consis s o a channel delay es ima o based on he EKF, which is illus a ed in Fig. 3., and designed o wo king p ope ly in acking [3]. Le ( ) be he ecei ed signal, c( ) he sp eading code assigned o he use o in e es and 2, he channel delay o be es ima ed. The i s block o he delay es ima o p ocesses he ecei ed signal in o de o imp o e he signal- o- in e e ence powe a io, ex emely low in CDMA en i onmen s. The second block is he EKF, which is esponsible o he es ima ion o he delay in oduced by he mobile channel. 11. DESCRIPTION OF THE CANCELLATION SCHEME Fig. 4 shows he in e e ence cancella ion scheme p oposed o a synch onous DS/CDMA sys em: cancella ion is ca ied ou be o e combining he di e en p opaga ion pa hs a he Rake ecei e . us0 slgnal ,A I, / 4 _- --7 h, '(2 73' ~~Smcommand Mobile L --- I Fig. 2. Synch onism scheme o a DSiCDMA synch onous sys em. Fig. 3. Synch onismscheme based on he Ex ended Kalman Fil e . Le G( ) be he Gold sequence assigned o he k- h use , L he numbe o p opaga ion pa hs a he ecei e inpu , M he numbe o a ms a he Rake ecei e , K he numbe o simul aneous use s in a gi en cell and S he a e age ecei ed powe in absence o mul ipa h. The "use k es ima o " in Fig. 4 compu es s,( ) (n=O,. . . ,L-1) om he ecei ed signal, ( ): whe e i , and dk a e espec i ely he es ima e o he n- h p opaga ion pa h complex ampli ude and he es ima e o he i- h ansmi ed bi , bo h co esponding o he k- h use , and ec ,( ) ep esen s a squa e wa e o m o du a ion T. Le n( ) ep esen he in e e ence gene a ed by o he use s in he adjacen cells. When pe ec synch onism be ween use s wi hin he same cell and pe ec es ima es o he channel impulsional esponse and ansmi ed bi s a e assumed, i can be shown ha ,,,( ) (i = 1,. . . ,K; m = 0,. . . ,M- 1) a e cancella ion akes he exp ession [2]: I . id 'I Fig. 4. Block diag am o he in e e ence cancelle . 69 Tha is, in a cellula mobile sys em limi ed by in e e ences, a linea cancelle such as his p oposed he e would be ideally limi ed in capaci y by he in e e ing signals coming om adjacen cells. In p ac ice, howe e , pe ec es ima es a e no a ailable and pe ec synch onism be ween use s canno be accomplished so, h oughou some ex ensi e simula ions, we will show in sec ion IV ha he pe o mance o he cancelle scheme in a eal en i onmen lies e y closed o his o he ideal case. 111. SIMULATION MODEL In o de o assess he pe o mance o he p oposed cancella ion scheme o a DSiCDMA CBPSK synch onous sys em based on he EKF delay es ima o , he comple e ansmission chain has been simula ed o K di e en use s wi hin a same cell. Such a chain is shown in Fig. 5, whe e sn( ) ep esen s he complex en elope o he signal ansmi ed by he k- h mobile: (3) The se o Gold codes assigned o each use , cL( ) (k=l, ..., K), has been ob ained om wo PN sequences o leng h N= 127 chips, gene a ed om he cha ac e is ic polynomials 211 and 277 (oc al) espec i ely [4]. Fig. 5 Block diag am o he simula ed DS/CDMA qynch onous sys em Le hk( ) be he low-pass equi alen impulsional esponse o he channel o he k- h use : The channel has been modelled wi h an exponen ial Powe Delay P o ile (PDP), wi h a Delay Sp ead no malised o he bi in e al equal o D,,=0.01, as usual in an indoo en i onmen . Conce ning he low-pass equi alen Dopple Powe Spec um o each channel p opaga ion pa h, he model "class" [SI, gi en by he exp ession hac been assumed, whe e d is he maximum Dopple equency. dT,=0.0005 has been se in ou simula ions o model he channel a ia ions in an indoo en i onmen . Finally, conce ning compu a ion o he bi and he channel impulsional esponse, which a e needed o pe onn cancella ion, a Rake ecei e and a il e ma ched o he co esponding Gold sequence ha e been conside ed espec i ely. A da a ame has been designed in o de o allow o es ima ion o he channel impulsional esponse du ing he ecep ion o he known bi s in he p eamble, as shown in Fig. 6, and es ima ion o he ansmi ed bi s du ing he ecep ion o he in o ma ion block. Q and D ha e been dimensioned o an indoo en i onmen (Q=lO bi s and D=90 bi s) acco ding o [6]. I . RESULTS AND CONCLUSIONS The pe o mance o he linea in e e ence cancelle has been analyzed unde he assump ion o an open-loop powe con ol and an EKF-based synch onism. Ex ensi e simula ions o he whole sys em ha e allow us o s udy he cancelle beha iou unde ealis ic si ua ions in e ms o capaci y and analyze he EKF-based delay es ima o in luence 70 I P eamble I In anla lon i~ - + ---___-___1 / Q bi s D bi s Fig. 6 Da a i ame s uc u e employed in he simula ion o he sys em on he cancelle pe o mance. The mean bi e o a e o a ixed numbe o simul aneous use s has been aken as igu e o me i o e alua e sys em pe o mance. Some ep esen a i e igu es ha e been buil om he esul s we ha e ob ained in ou simula ions. Among hese igu es, we ha e chosen Fig. 7 and Fig. 8 ha illus a e he mean BER in e ms o he sys em capaci y o an ideal synch onous access and o a synch onous access based on he EKF delay es ima o . The numbe o a ms in he Rake ecei e has been assumed o be M = 2 in bo h cases. F om he wo o hem, we can conclude ha he use o he cancelle s uc u e esul s in an impo an capaci y imp o emen wi h ega d o he single-use ecei e , i.e. a simple Rake ecei e , which may be e en g ea e in he p esence o channel encoding. In addi ion, he synch onism scheme based on he EKF is obus enough wi h e y li le impac on he cancelle pe o mance. This is he eason why 0.1 1- I -+-wi h ca celle J 15 25 35 45 55 NUMBER OF c,iYJiTANFOLIS IJSERS Fig. 7. Bi E o Ra e a he ou pu o he cancelle o an ideal synch onism. w c 13.81 CY 0 E CL w - G7 _. ~.m! LL z +no cancelle D. Mbal i &wi !] cancelle 5 15 25 35 45 55 NUMBER OF SIMULTANEOUS USERS Fig. 8. Bi E o Ra e a he ou pu o he cancelle o an EKF-based synch onism. i seems o be a good candida e o synch onising he di e en use s a he base s a ion o a synch onous sys em in Rayleigh mul ipa h en i onmen s. V. REFERENCES S. Ve du, "Minimum P obabili y o E o o Asynch onous Gaussian Mul iple-Access Channels", IEEE T ans. on In o ma ion Theo y, Vol. IT-32, Janua y 1986. P. Diaz, R. Agus i, "Analysis o a Linea In e e ence Cancelle in Cellula DSiCDMA Sys ems", in P oceedings o he 43 d IEEE VTC, June 1994. P. Diaz, D. Henche, R. Agus i, "A PN Code Delay Es ima o Based on he Ex ended Kalman Fil e o a DSiCDMA Cellula Sys em", in P oceedings o he 4 h In e na ional Symposium on PIMRC, Sep embe 1993. R. de Gaudenzi, C. Elia, R. Viola, "Bandlimi ed Quasi-Synch onous CDMA: A No el Sa elli e Access Technique o Mobile and Pe sonal Communica ion Sys ems", IEEE Jou nal on Selec ed A eas in Communicu ions, Vol. 10, No. 2, Feb ua y 1992. Cos 207, "Digi al Land Mobile Radio Communica ions", In o mu ion Technologies und Sciences (CECJ, 1989. P. Diaz, R. Agus i. "On he In luence o he Channel Impulsional Response Es ima ion on he Pe o mance o a Linea In e e ence Cancelle o a DSiCDMA Sys em", in P oceedings o he 5 h In e nu ional Symposium on PIMRC, Sep embe 1994. 71